1. Field of the Invention
The present invention relates generally to telecommunications and, more particularly, to evaluating the subjective quality of packetized signals.
2. Description of Related Art
Packet-switched communication networks are being used increasingly for transporting real-time interactive telecommunication signals such as telephony, video telephony, multimedia communication, teleconferencing, and multimedia teleconferencing. Such packet-switched networks are also being used increasingly for distributing and delivering static stored content and live content such as, for example, video clips, live video, movies, audio, music, and radio. A fundamental difficulty in supporting transmission of these types of signals is ensuring that signal quality, as perceived by end-users, is at required levels. To achieve this signal quality at the end user device, the packet-switched network should be designed to provide required subjective quality levels. Mechanisms should also be in place to ensure that required quality levels are maintained on an ongoing basis.
To engineer a packet-switched network that meets required quality levels in the telecommunication signals mentioned above, it is necessary to determine and understand how subjective quality depends on various combinations of network-level quality-of-service (QoS) parameters, type of coder/decoder (codec), packetization method, buffering mechanism, play-out algorithm, and other related parameters. QoS parameters may characterize the performance of the transport service provided by a network. For example, QoS parameters may include packet loss percentage, packet delay, and packet delay jitter.
FIG. 1 illustrates one approach that has been proposed, namely building a test telecommunications network 110 in a laboratory environment 100 and evaluating signal quality directly using human subjects 120. A difficulty with this approach is the expense and time associated with using human subjects, especially when there are many different experiments (e.g., combinations of the network parameters listed above) to be carried out. Constructing and operating test network 110 in lab environment 100 can also be expensive. It may also be time-consuming and expensive to modify test network 110 for different experiments. Another difficulty with the approach illustrated in FIG. 1 is that experimental results generally are not reproducible, because subjective evaluation is carried out using real human subjects 120 whose opinions of signal quality may vary with repetitions of the same experiment.
FIG. 2 illustrates another approach that has been proposed, namely using test equipment 210 that applies objective methods to measure quality of signals from test network 110 in laboratory environment 100. In contrast to using human subjects 120, the objective measurement scheme in FIG. 2 applies a machine-executable algorithm within equipment 210 to determine the quality of a received signal as compared to an original transmitted signal.
An example of an objective method is objective quality measurement based on Perceptual Speech Quality Measure (PSQM). PSQM is a computation-based method to objectively measure perceived quality of a speech signal, and is specified in the International Telecommunication Union-Telecommunication (ITU-T) P.861 recommendation. This PSQM method was originally developed for evaluating subjective quality of speech codecs, but it is now also being applied to objectively measure voice-call quality across packet-switched networks. Examples of commercially available test equipment that use PSQM include the Abacus™ system from Zarak Systems Corporation, the VoIP Test System™ from Hammer Technologies, and the 935AT Telecom Test Set™ from Sage Instruments. Although objective signal quality measurement equipment may be used to replace human subjects in evaluating signal quality, applying such an approach to test network 110 in lab environment 100 can still be time consuming and expensive when there are many experiments to be carried out.
Thus, there is a need in the art to subjectively measure signal quality over a packet-switched network having variable parameters in a quick and inexpensive manner.